"Going to the sun road" is an easy way to see a lot of Glacier. A scenic 2 hour drive (1 hour out, 1 hour back) with many viewpoints, photo ops, and trailheads along the way. If you decide to hike, remember it's grizzly country :) You can rent bear spray at the airport. There are also tours leaving from Whitefish for a fully guided experience. Vehicle reservations are no longer required!
A really chill family-friendly option if you want to get out on the lake. You can rent SUPs and kayaks here, they also have towels and sunscreen for purchase. There's a big swimming area and shore to relax on the beach or throw the frisbee.